std.os: remove special case for haiku
std.os: remove special case for haiku

This is an accident from a merge conflict introduced in
7edb204edfa41e11776ac009da5a20fb1c907f5f.

The new pipe2 code I believe is supposed to work for all posix-like
systems. If haiku needs special handling here, it should be
re-introduced.

zig-lang/zig,zig-lang/zig,andrewrk/zig,andrewrk/zig,zig-lang/zig